1. Field of the Invention
The invention relates to increasing the size of microdrops produced by ink-jet technology to a size which is significantly larger than the orifice diameter; especially solder microdrops for use in the electronic industry.
2. Background of the Art
Although this invention is applicable to the dispensing of various liquids, it has been found particularly useful in the environment of dispensing very small solder balls to very small soldering areas. Therefore, without limiting the applicability of the invention to "dispensing very small solder balls to very small soldering areas," the invention will be described with reference to that environment.
In a high density electronic manufacturing process, semiconductor integrated circuit chips are bonded to a substrate by a solder reflow process. This is commonly referred to as a "flip-chip" process in which solder bumps are placed on pads of the integrated circuit chip or other chip, then turned over and matched with solder wettable terminals or connect pads or bond pads with or without solder, on a substrate. Such processes are described in U.S. Pat. No. 5,229,016, U.S. Pat. No. 5,193,738, U.S. Pat. No. 5,415,679, and U.S. Pat. No. 5,643,353, all incorporated herein by reference. These references discuss various ways of producing solder bumps and interconnections for electronic devices.
Current flip-chip assembly processes typically use 100-125 .mu.m diameter bumps on pads of similar dimensions. Solder droplets produced by known solder jet systems are typically 25-75 .mu.m in diameter with the actual value being largely determined by the orifice diameter of the solder jet device in use. Larger diameter solder bumps are needed. Attempts to operate solder jet devices with orifice diameters greater than 75 .mu.m to make larger drops have been unsuccessful due to instability of the drop formation process. In addition, it would be highly desirable to be able to select in real-time solder droplet diameter, over a fairly broad range, to be dispensed from a given solder jet device such as mentioned in the patents set forth above.
One approach to producing solder bumps of the larger diameter needed for microelectronic fabrication is to dispense multiple droplets onto a single substrate site in order to overcome the 75 .mu.m limitation. This has actually been done in private experiments whereby eight or more nominally 50 .mu.m diameter droplets have been printed onto an integrated circuit pad 125 .mu.m in diameter where they are solidified in a tower-like mass approximately the same diameter as the pad. Although the drop-to-drop rate in these experiments was fairly high (248 Hz), because of the requirement that the drop impact onto the same location, the printhead had to be stationary during dispensing, and only three to four bumps per second were produced. This is far less than the production rate required to provide economical production. Thus, multidroplet dispensing to a site inherently limits the throughput of a printhead system. To maximize throughput, a single droplet must be dispensed per pad, large enough and allowing the printhead to dispense while it is moving.
In the field of ink-jet technology, attempts have been made to modulate the drop size produced by drop-on-demand type ink-jet printheads in order to improve the image quality.
Such a procedure is disclosed in U.S. Pat. No. 5,461,403 which is incorporated herein by reference. Thus, disclosure illustrates the way a conventional unipolar pulse waveform is applied to the piezoelectric material in an ink-jet device. The voltage in a unipolar pulse rises rapidly from an initial voltage to a first voltage where it is held for a primary dwell time and then rapidly returned to the initial voltage. It illustrates that conventionally, drop volume can be increased to a maximum by varying the primary dwell time, however, velocity of the droplets follows almost exactly the volume curve. Thus any attempt to modulate the size of droplets is cursed with a corresponding change in drop velocity. As a result, droplet placement accuracy is lowered significantly before the droplet volume is significantly decreased. Without droplet placement accuracy, it is said the usefulness of such technology in the printing arts is minimal.
The invention disclosed in the reference is a method of modulating drop size by varying the first and second dwell time in a bipolar waveform. The voltage increases from an initial rest voltage to a first rest voltage. After the primary dwell time, the voltage traverses past the initial rest voltage in an "echo portion" to a second rest voltage is held for an echo dwell time before it is returned to the initial rest voltage. Primarily by adjusting the amount of echo dwell time the references shows it is possible to partially separate the change in volume from the resulting velocity of the droplets as a form of real-time droplet volume modulation. However, these efforts are directed to producing droplets smaller than the orifice diameter in a demand mode ink-jet printing device for image production quality improvements. The initial transition time in the voltage rise from initial rest voltage to the first rest voltage remains conventional at about 1 to 5 microseconds. There is no incentive for conventional ink-jet technology to increase the length of waveform in an ink-jet for printing because it could decrease the operating frequency of the device. Decreased operating frequency would affect the rate at which printing can be done.